1. Field of the Invention
The present invention relates to a watch equipped with a structure for holding the crown to prevent it from being inadvertently rotated and to a crown used in such a watch.
2. Description of the Prior Art
In some watches such as wristwatches, the crown is locked by making use of threaded engagement to prevent inadvertent rotation of the crown when carrying the watch (the structure or function is referred to as threadedly locking, threadedly locking structure, or threadedly locking function in the present specification).
In the threadedly locking structure, a stempipe is mounted to a barrel in which a watch movement is incorporated. The internal thread of the crown which is fitted over a barrel outside protruding portion is threadedly engaged to an external thread formed on the outer surface of the barrel outside protruding portion of this pipe. In this way, during normal times, the crown is threadedly engaged to the outer surface of the barrel outside protruding portion. Thus, threadedly locking is achieved, preventing inadvertent rotation of the crown. Also, when the watch movement is manipulated, the crown is threadedly disengaged from the external thread and then the crown is pulled and moved. Under this condition, the stem located inside the stem pipe can be rotationally manipulated via the crown.
The crown of the conventional watch equipped with such a threadedly locking structure consists of a single part. In particular, it is composed of an inner cylindrical body and an outer cylindrical body integral with it. The inner cylindrical body is a portion inserted into a stem pipe mounted to a barrel from outside of the barrel. The outer cylindrical body is shorter than the inner cylindrical body and covers an end part of this inner cylindrical body. An inner thread portion is formed in the inner surface of this outer cylindrical body. The crown is threadedly locked by bringing this inner thread into threaded engagement with an external thread formed on the outer surface of a barrel outside protruding portion of the stem pipe and bringing an end surface of the outer cylindrical body into abutment with a barrel outer surface (see, for example, Japanese Utility Model Application No. 7-26792).
The crown of the integrated structure described in Japanese Utility Model Application No. 7-26792 has a wall portion connecting its outer cylindrical body and inner cylindrical body integrally. This wall portion hinders insertion of a tapping tool beyond a certain extent when an internal thread is machined on the outer cylindrical body using the tapping tool such as a tap.
In this way, in threading machining for forming an internal thread on the outer cylindrical body of a crown, a care needs to be taken to prevent the tapping tool from striking the wall portion. Therefore, it is difficult to machine the crown. Consequently, this machining is disadvantageous in terms of cost.
Since the workability (machinability) of the threading machining is poor as described above, the accuracy at which internal threads are machined varies widely. As a result, there may occur a combination in which the accuracy of threaded engagement between the external thread of the stem pipe and the internal thread of the crown is low. In this case, as the crown is rotationally manipulated, the life of the threadedly engaging portion may be shortened.
Furthermore, because of the size of the crown that is a small part, 3 to 4 thread ridges can be formed at maximum on the internal thread formed on the outer cylindrical body. During manufacturing, the tapping tool makes the internal thread without passing through the outer cylindrical body. Therefore, those portions of this internal thread which are close to the wall portion are incompletely threaded portions. Consequently, as threadedly locking is done by bringing the external thread of the stem pipe and the internal thread of the crown into mesh each other, the incompletely threaded portions of the internal thread portion deform the external thread. This may lead to a decrease in the life of the threadedly engaging portion.
It is an object of the present invention to provide a watch which is capable of improving the life of the threadedly engaging portion between a stem pipe and a crown and which permits easy and accurate machining of an internal thread on the crown, as well as the crown used in this watch.